1. Field of the Invention
This invention relates generally to technology of ion and plasma sources, and more particularly to Hall-type ion sources producing high-current ion beams that can be utilized in thin film processing technology. Historically, thrusters, or accelerators of ions were utilized for space application to move, or stabilize space satellites since early 70-ies. Ion sources that can be considered a spin-off of electric propulsion thrusters have the same operational principles. However, they do not need to be light and efficient as thrusters; they need to accelerate ions, produce high ion beam currents with regulated ion beam mean energy, be efficient in vacuum etching, deposition, in assisting to certain physical processes involving interaction of sputtered particles with surface of a substrate. Hall current in ion and plasma sources is a result of interaction of electrical charge carriers—electrons and ions caused by separate direction of electric and magnetic fields. Change of conditions leading to a value of charged particles density by a value and geometry of magnetic field, shape of electrodes and discharge channel leads to separation of charged particles caused by particles different trajectories and appearance of Hall current, which is directed to a normal to vectors of electric field, E and magnetic field, B.
2. Description of the Prior Art
For technological applications, one of Hall-type ion sources was introduced in 1989 in U.S. Pat. No. 4,862,032 by Kaufman, et al., which in 2003 was modified in form of a modular ion source by Kaufman, U.S. Pat. No. 6,608,431 B1. This ion source also considered as gridless ion source with a discharge chamber determined by a conical shape of a hollow anode, and also called an end-Hall ion source with a circular discharge region and only an outside boundary. In 2003 Sainty obtained a U.S. Pat. No. 6,645,301 B2 called “Ion Source”. This patent has a very similar concept and design of a Hall ion source as in U.S. Pat. No. 4,862,032 by Kaufman et al., and practically the same conical shape of a hollow anode, with some minor changes such as a gas distributing system (reflector), which in Sainty's patent is at an anode potential. In Kaufman et al., U.S. Pat. No. 4,862,032, and in Kaufman U.S. Pat. No. 6,608,431 B1 a gas distributing system is at floating potential. These publications are incorporated herein by reference.
In general, among gridless ion sources there are two most common types of ion sources, both also called as Hall ion sources: a closed drift ion source with annular discharge chamber and an end-Hall ion source with a circular discharge chamber occupied mostly by a hollow anode of a conical shape. However, for a distinction, the first one will be called a closed-drift ion source and the second one, an end-Hall ion source. Both types of ion sources utilize a Hall effect that playing a major role in acceleration of ions.
Ion sources with closed electron drift have been utilized from early seventies, since appearance in space of first Russian thrusters with closed electron drift in 1972. A detailed review of closed drift ion sources/thrusters features, which is applied to any Hall-current sources, is described by Zhurin, et al., in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. This publication is incorporated herein by reference.
Such ion sources operate in a following way. Working gas supplied into a channel close to anode is ionized by electrons moving under impact of electric field from cathode to anode in a radial magnetic field. In a traditional performance, an ion source comprises of anode, cathode, discharge chamber with accelerating channel, a magnetic system with magnetic poles, magnetic means provided by electromagnetic coils, or permanent magnets, a central core and a magnetic path. A magnetic system is designed in a way that in an annular accelerating channel a mainly radial magnetic field is realized. An electric potential is applied between anode and cathode, and an electric field in a discharge channel is directed approximately parallel to an ion sources axis. A working gas, which must be ionized, is supplied into a discharge channel through anode. Though it is possible, and used frequently, working gas is applied through a separate gas distributor, regularly placed under anode area, and from this area a working gas is directed into an anode area.
In closed drift ion sources, there are two main types of ion sources distinguished with length and material of a discharge channel. One type, called a magnetic layer ion source, which has a discharge channel length that is greater than its width and usually has discharge channel made of dielectric material; though, there are types of a magnetic layer ion source that discharge channel walls made of a conducting material. The other type, called an anode layer ion source has a discharge region length that is less than its width and its walls made of conducting material. Both sources have very similar characteristic performance with some non-fundamental differences.
In Hall ion sources a magnetic field value is selected in such a way that Larmour radii for electrons, rLe and ions, rLi calculated through energy corresponding to applied potential difference satisfy to a condition: rLe<<L<<rLi, where L is a characteristic dimension of an acceleration region in an ion source's discharge channel. In Hall ion sources a cyclotron frequency of electrons, ωe must be greater than a frequency of electron collisions, ν with other particles and discharge channel walls, i.e. ωe>>ν=1/τe, where τe is an average time between electron collisions with other particles and discharge channel walls. That is why so-called Hall parameter, ωeτe that utilized for a characterization of electron magnetization is ωeτe>>1.
The condition for magnetization of electron component in plasma (ωeτe>>1) and, at the same time, an ion component is not magnetized (ωiτI<<1) means that a determining process in closed drift ion sources is an ion current motion in a discharge region, and an electric field is “suspended” on a magnetized electron component. In end-Hall ion and some other types of ion sources, electrons in certain areas of discharge channel occupied by plasma (at exit in end-Hall ion sources) can be only partially magnetized. In such cases, Hall effect leads to a change of direction of electron motion and to a corresponding change of volumetric forces forming and accelerating plasma flow. Hall parameter, βe determines a relative value of a Hall electromotive force and influence of a magnetic field on plasma electric conductivity, σ:βe=EHall/E==|j×B|/[(1/σ)|j|en]=σB/en=ωeτe. With the increase of a Hall parameter, ωeτe a motion of charged particles across of a magnetic field becomes more difficult and particles begin to drift with a velocity, ν=(E×B)/B2, or in mutually orthogonal fields, E and B. A drift velocity can be determined through a ratio of electric and magnetic fields, νdrift=E/B.
In closed-drift and in certain area of end-Hall ion sources a primary motion of electrons is in azimuthal direction. Because an azimuthal electron velocity is significantly higher than a longitudinal electron velocity component, electron trajectories are almost closed. And this determines a name of a first one: a source (or a thruster) with a closed electron drift.
End-Hall ion sources also can be called sources with closed electron drift, however, a situation here is different. A Larmour electron radius, rLe is smaller than L, but only at a gas distributor/reflector, where a magnetic field usually is quite strong. In existing end-Hall ion sources this value is from 600 to 1000 G. And a magnetic field in this area has mainly an axial direction. Magnetic field decreases significantly from a gas distributing area and at the discharge channel's exit it is only about 50–60 G. In general, in existing closed-drift ion sources, a magnetic circuit is designed in such a way that a magnetic field increases from anode to a discharge channel's exit. The best efficient operating closed drift ion sources have a magnetic maximum optimum value of about 200–450 G for Argon and 450–750 G for xenon. In end-Hall ion sources the applied magnetic field lines are mainly axial at the top of gas distributing system-reflector and are mainly radial at exit of discharge channel, close to an external magnetic pole. The end-Hall ion sources have a negative magnetic gradient and the closed-drift ion sources have a positive magnetic gradient in a discharge channel.
In a process of ion sources operation, a motion of electrons takes place from cathode to anode region and to anode itself, this motion is accompanied by collisions with atoms of working material, with ions, with discharge chamber walls and due to discharge oscillations. As it was above noted, ions are practically not magnetized and they move mainly along applied electric field and are accelerated in this field. A flow of ions “captures” necessary number of electrons produced by an external source of electrons, so these ions become neutralized and together they develop a plasma flow.
Since electrons drifting in an azimuthal direction neutralize an ion volumetric charge in an ion source's discharge channel, in closed drift and end-Hall ion sources there is no limit for an ion beam current by a space electric charge. This feature is a significant advantage of closed-drift and end-Hall ion sources in comparison with electrostatic or so-called gridded ion sources.
Because electrons in magnetic field are moving along magnetic field lines relatively free before their collisions with neutral atoms, in a first approximation it is possible to consider the surfaces going through magnetic field lines in an azimuthal direction as surfaces of equal potential. This is one of major ideas in a possibility and necessity to control and focus an ion flow through a selection of a corresponding configuration of magnetic field lines.
In both types of ion sources, end-Halls and closed-drift types it is necessary to have a source of electrons to start a discharge and ionization of a working gas. Analysis of discharge at low pressure (rarefied regimes, P≦1 mtorr) and moderate discharge voltages (50–1000 V) and currents (1–20 A) shows that from about 50 to 350 V a discharge represents itself so-called a non-self-sustained discharge and from about 350 V and higher it represents a self-sustained discharge. It means that, in order to maintain a discharge in a discharge channel of these ion sources, it is necessary to provide a source of electrons at discharge voltages under about 350 V. For ion sources with operating discharge voltages over 350 V it is necessary to start discharge and after its beginning it can maintain itself providing electrons from small sparks in vacuum chamber and ion source's discharge chamber itself.
Hot filaments and hollow cathode electron sources are generally used as cathodes in closed drift and end-Hall ion sources. Hot filaments, which utilize a tantalum and tungsten wire, can produce electron currents from about 0.1 A to about 30 A. Modern hollow cathode-neutralizers make possible to obtain electron currents from 0.5 A to 75–100 A with a flow of working material that in 10–50 times lower than in an ion source itself. However, there are other types of cathodes that can be utilized for neutralization of Hall-current ion source's ion beam, such as a “plasma bridge” and a “cold hollow cathode”, a device utilizing a glow discharge in a longitudinal magnetic field.
One of the most distinguished features of a U.S. Pat. No. 4,862,032 by Kaufman, et al., as it was above mentioned, is that a magnetic field strength decreases in a direction from anode to cathode: page 2, lines 55–59; page 10, claim 1, lines 60–64; page 11, claim 4, lines 55–59. This provision is very distinct and emphasized through the whole patent and makes it, as was above mentioned, an ion source with a negative gradient of magnetic field. And this particular feature, a decreased value of a magnetic field along an ion source's discharge region, substantially reduces a range of operation conditions of an end-Hall ion source, especially in the range of discharge voltages over 300 V, and can be considered as a major shortcoming of that type of ion source.
There are other important shortcomings of existing end-Hall ion sources caused by a negative magnetic gradient of magnetic field in a discharge channel of such an ion source. These are the following shortcomings that necessary to mention:
a) An ion beam current, Ib is only about 20–25% of a discharge current, Id or Ib/Id≈0.2–0.25, because the conditions for efficient ionization of atomic particles in a discharge chamber do not exist;
b) Ratio of equivalent mass flow current, Im (Im=ema/M, where e is electron charge, ma is working gas mass flow, and M is working gas atomic mass) of consumed mass flow to an ion beam current, Im/Ib≧1.2, which means that at low discharge currents (Id<5 A) and high mass flows, the most portions of working gas is not utilized efficiently. However, at higher discharge currents (Id>5 A)Im/Ib≦1.2, meaning that a certain portions of working gas is double ionized particles. For good efficient operation of ion source an ion beam current and equivalent mass flow current must be close to each other, or Im/Ib≈1.
b) A gas-distributor, called sometime as a reflector, which is usually under a floating potential (it assumes plasma potential of a discharge channel), as in U.S. Pat. No. 4,862,032 by Kaufman et al., or at anode potential, as in U.S. Pat. No. 6,645,301 B2 by Sainty, has very short time. Its central part is bombarded by energetic ions that are a part of a whole anode flow that, in general, is moving outside of a discharge chamber but some substantial parts are moving in opposite direction, to a gas distributor. In result, in a central part of a gas-distributor/reflector after about 10–20 hours of operation at discharge currents, Id≧5 A, and Vd≈150 V, there can be observed either a hole, or a big chunk of material is removed by sputtering from a central part of a gas-distributor, depending on discharge parameters. This shortcoming feature is not only forces to frequently substitute gas-distributors, but their sputtering contaminates process's targets and substrates by a gas-distributor's material, because after an ion bombardment sputtered particles move out of discharge chamber and become deposited all over a vacuum chamber and other parts.
c) Existing end-Hall ion sources have problems in operation at discharge voltages over 300 V. High amplitudes of discharge current and oscillations are developed and prevent normal discharge process making a range of operation insufficient for certain necessary conditions in many cases in technology: discharge currents should be over 10 A and discharge voltages should be 1000–1500 V. Such operation conditions, with high discharge currents and voltages significantly enhance sputtering and deposition. The developed oscillations are explained by a configuration of a magnetic field that decreases from anode to cathode, or a negative gradient of a radial magnetic field.
d) An ion beam coming out of end-Hall ion source is very divergent: due to a decreasing magnetic field, and due to a design of an end-Hall ion source that has an external magnetic pole piece placed quite wide following an anode's conical shape.
Thin film deposition in many cases requires ion assisting of low energy ion sources. Magnetic field configuration with strong axial component of magnetic field makes possible for end-Hall type ion sources to operate at discharge voltages, Vd lower than 100 V, at 40–50 V with Argon and at 20–30 V with Xenon. Closed drift ion sources with strong radial component of magnetic field at ion source's exit make possible to start discharge at voltages from 100 V to over 1000 V with practically all working gases. A combination of both types of ion sources helps to extend a range of operating conditions.
A B-E (magnetic-electric fields) discharge should effectively combine several functions: to prevent direct motion of electrons from cathode to anode, forcing electrons to drift to anode in closed loops, to generate and accelerate ions in a discharge channel. In general, a B-E discharge always has oscillations and instabilities of main operating parameters: discharge current, Id and voltage, Vd. Oscillations and instabilities were found by researchers from the beginning of studying closed-drift and end-Hall ion sources and thrusters. However, most instabilities and oscillations actually is a part of normal operation of ion sources. And, a presence of oscillations in plasma with intensity that does not exceed certain critical value, even if they lead to a partial decrease of efficiency of ion production, can provide stable operation of ion source in regimes that could not be realized otherwise. However, instabilities and oscillations that become about 100% of discharge current, Id and voltage, Vd can destroy normal discharge and extinguish ion source operation.
In general, there are many different types of oscillations accompanying B-E discharge. Among them there are several groups of the most prominent and important oscillations that can disrupt normal operation of an ion source. More detailed information about oscillations in B-E discharge can be found in a mentioned article by Zhurin, et al., “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on Page R1. These oscillations are:
Contour oscillations are longitudinal oscillations with a characteristic frequency of 1–30 kHz. Their mechanism is due to instability of ionization region in a discharge area. These oscillations are most intense oscillations and at the regimes with developed oscillations of this type there are observed a 100% modulations of discharge parameters. Contour oscillations can be suppressed by a correct configuration of a magnetic field, discharge voltage, working mass flow, and parameters of power supply.
Ionization oscillations have maximum frequencies in a range of tens to hundreds of kHz. These oscillations are caused by an azimuthal wave traveling in a direction of electron drift; they are connected with an ionization wave of a working material. This instability appears beginning from a certain critical value of a parameter IdB/ma (where Id is a discharge current, B is a magnetic field, and ma is a working material mass flow); with a growth of this parameter an amplitude of a discharge voltage increases achieving 15–25% of a nominal discharge voltage. Ionization instability can be decreased substantially with a higher discharge current, when a regime of complete ionization is observed.
Flight oscillations are characterized by a broad spectrum of frequencies in a range of 100 kHz up to 10 MHz and they correspond to an ion flight time through a discharge channel. Amplitude of flight oscillations can achieve 20–30% of value of discharge parameters. Plasma potential and particles density are pulsed along an ion source synchronously; however, these oscillations are non-symmetrical along azimuth, and this leads to development of alternating electric fields. Plasma turbulence increases with appearance of flight oscillations.
Spoke-type oscillations. Every type of ion source always has a certain range of optimum operation parameters such as discharge current, Id and voltage, Vd, working material (gas) mass flow, ma, magnetic field, B. Before an ion source starts operation in optimum regime, at a low-voltage part of volt-ampere characteristics of discharge there always takes place an ionization instability of a spoke type that rotates in an azimuthal direction with a constant velocity, vφ≈cvEz/Br, where cv is a constant in a range of 0.4–0.8. A structure of this oscillation wave (20–60 kHz) is characterized by an increased electron concentration, ne.
High-frequency oscillations are typically in a range of 1–100 MHz. They are hybrid azimuthal oscillations developed in an ion source with a negative gradient of a magnetic field. These oscillations are harmful for end-Hall type ion source in a whole discharge channel and in closed drift ion sources they are important at an ion source's exit, where magnetic field changes from positive to negative gradient.
The most intensive are contour oscillations. These oscillations are also a problem for end-Hall type ion sources, where a magnetic field decreases in a discharge region. Such oscillations lead to a substantial divergence of ion flow, to sputtering of a discharge channel, to unnecessary discharge channel's heating.
Due to great importance for solution of oscillation problem for optimization of processes in closed drift and Hall-type ion sources, it is necessary to use different ways for stabilization and suppression of instabilities. Besides of above mentioned article by Zhurin, et al., “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8, beginning on page R1, there are many other studies devoted to oscillation problem in ion and plasma ion sources/thrusters such as Zhurin, et al., “Dynamic Characteristics of Closed Drift Thrusters”, published at 23rd International Electric Propulsion Conference, Sep. 13–16, 1993, IEPC-93-095, beginning on page 1, and Randolph, et al., “The Mitigation of Discharge Oscillations in the Stationary Plasma Thruster”, published at 30th AIAA Joint Propulsion Conference, Jun. 27–29, 1994, beginning on page 1. These publications are also incorporated herein by reference.
A fundamental criterion for suppression of instabilities in Hall-current closed drift ion sources/thrusters was introduced by Morozov in article “On Equilibrium and Stability of Flows in Accelerators with Closed Electron Drift” in Russian publication “Plasma Accelerators”, Proceeding of 1st All-Union Conference on Plasma Accelerators, Moscow, Publishing House “Mashinostroenie”, 1973, beginning on page 85, that in Hall-current ion sources/thrusters with closed electron drift, in order to have a flow with suppressed oscillations, it is necessary to utilize in a discharge channel a magnetic field with a positive magnetic gradient: ∂Br/∂x>0. Morozov's publication is incorporated herein by reference. In above mentioned article by Zhurin, et al., in an article “Physics of Closed Drift Thrusters” in Plasma Sources & Technology, Vol. 8, on page R8 there is information about this stability criterion.